Type B1 Thymoma (Lymphocyte-rich) : Microscopic
Type B1 thymomas usually show a lobular architecture with fibrous bands arising from the capsule dividing the tumor into lobules or nodules. The lobulation may sometimes be absent. The tumor resembles normal thymus and shows a predominance of dark blue cortical areas and a few pale islands of medullary differentiation. The cortical areas are composed of scattered neoplastic epithelial cells in a background of a dense infiltrate of non-neoplastic immature T-cells. The presence of tingible-body macrophages surrounded by densely packed lymphocytes has created a starry-sky appearance in this image. The epithelial component is inconspicuous at this magnification.